The present invention relates to a ribbon protector which is installed in a printer, wherein printing is performed by a printing head facing a platen and performing a spacing movement for printing, and more particularly, in a printer which has a platen gap adjustment function for adjusting a distance between the platen and the printing head with regard to the thickness of paper set into the printer. The ribbon protector is installed between the paper and an ink ribbon for protecting paper from contamination by the ink ribbon.
The present invention also relates to a platen gap adjustment device which is intended for adjustment of the distance between the surface of paper which is laid onto a platen and the tip of a printing head in a printer which carries out the printing operation by means of the above-mentioned printing head which performs spacing movements with regard to the platen.
In order to protect a printer in which printing is performed by a printing head facing a platen and performing spacing movement, from a decrease in the quality of printing, breakage of the head pin, contamination of paper by an ink ribbon, and from inaccurate spacing movements of the carriage, it is necessary to maintain a predetermined distance between the surface of the paper and the tip of the printing head in relation to variations in the thickness of the paper.
FIG. 1 is a perspective view of a known printer which incorporates a paper thickness detection function and a platen gap adjustment function.
In this drawing, reference numeral 1 designates a platen, and 2 designates a printing head which is located in front of platen 1 and fulfills a printing function by being moved with a carriage 4 which performs spacing movements along a carriage shaft 3.
Both ends of carriage shaft 3 are fixed to cams 5. Reference numeral 6 designates a pulse motor which rotates the above-mentioned cams 5 through gears 7a and 7b. The printer is also provided with a spring 8, a slit disk 9, and a photoelectric sensor 10 which detects pulses produced by rotation of the above-mentioned slit disk 9. Through slit disk 9 and photoelectric sensor 10, it is possible to determine the moment when pulse motor 6 is stopped.
Reference numeral 11 designates a paper set between platen 1 and printing head 2, and 12 designates an ink ribbon. In the known device, a ribbon protector, which is intended for protection against contamination by a ribbon 12, does not have any relation to the platen gap adjustment function and therefore is not shown in the drawings.
The mechanism described above maintains a constant distance between the surface of the paper 11 and the tip of the printing head 2 at different thicknesses of paper 11 inserted into the printer in the following manner.
First of all, when paper 11 is guided around platen 1, the pulse motor 6 turns in a forward direction, and through gears 7a and 7b, rotates cams 5 rigidly attached to carriage shaft 3, so that carriage shaft 3 is turned and moved in a direction of withdrawal from platen 1 which is indicated by an arrow labelled X in FIG. 1.
At the same time, carriage 4 and printing head 2 installed on carriage 4 are also moved in a direction indicated by the arrow labelled X away from platen 1 and stop in a predetermined position.
Pulse motor 6 is then turned in a reverse direction which is opposite to the direction indicated by the arrow labelled X, i.e., so that print head 2 approaches platen 1, and abuts the platen 1 clamping paper 11 and ink ribbon 12.
Now, the printing head 2 cannot move further toward platen 1.
At this moment, slit disk 9 is at a halt, photoelectric sensor 10 does not generate pulses, and the system detects a stop state of pulse motor 6. From this point, for the second time, pulse motor 6 rotates in the forward direction only for a predetermined amount, so that a specified gap is obtained between the surface of paper 11 and the tip of the printing head 2.
Because normally the above-mentioned predetermined amount of rotation is constant, the above-mentioned specified gap between the surface of paper 11 and the tip of printing head 2 can be maintained constant, irrespective of the thickness of paper 11.
However, as shown in FIG. 2, the pulse motor 6 continues to rotate even after contact of the printing head 2 with the surface of the paper 11 because even after the contact, the clamped paper 11 and ink ribbon 12 can be further compressed and the carriage shaft 3 and the carriage frame FR can be bent, until the reaction of the shaft 3 coincides with the pulse-motor torque, when the pulse motor 6 is stopped. That is the pulse motor 6 stops only when it overruns the zero-gap condition by a certain value .DELTA.N.
Because the value of overrun .DELTA.N depends on the pulse-motor torque, variation of the initial torque of the pulse motor 6, reduction of the torque because of an increase in temperature, fluctuations of voltage, or the like causes changes in the value of overrun. As a result, the return stroke of the printing head 2 is unstable.
Moreover, in the device of the type described above, the platen gap is adjusted indirectly. That is, the gap adjustment is made without taking account of the distortions of the carriage shaft 3. As a result, the accuracy of the adjustment is low.
Furthermore, because the platen gap is adjusted through the rotation of the carriage shaft 3, the mechanism has increased overall dimensions, as it requires that rotary means such as gears 7a and 7b, as well as drive means, such as a motor 6 for driving the above-mentioned rotary means, be installed on the end of the carriage shaft 3.
A further problem associated with the prior art device is its high cost resulting from the use of the slit disk 9 and photoelectric sensor 10.